This invention relates to a process for producing high impact and weather resistant graft copolymers, and more particularly it relates to a method of producing graft copolymers with excellent impact and weather resistance according to a two-stage process by graft copolymerizing a rubber-like polymer such as ethylene-propylene copolymer rubber, ethylene-propylene-non-conjugated-diene copolymer rubber or butyl rubber with an aromatic vinyl monomer and a polar vinyl monomer copolymerizable therewith.
Three-component graft copolymer resins obtained from copolymerization of aliphatic vinyl monomers, diene rubber and aromatic vinyl monomers, such as for example acrylonitrile-butadiene-styrene terpolymer resin popularly called as ABS resin, are widely used as thermoplastic resins with excellent impact resistance. However, since this kind of resins contain as a molecular constituent a rubber component having carbon-carbon double bonds such as polybutadiene rubber or styrene-butadiene copolymer rubber, they have the vital defect in that they are very susceptible to oxygen, ozone or ultraviolet rays and hence if they are left exposed in the air, particularly in the outdoors, they are bound to suffer excessive deterioration of mechanical properties such as impact strength and elongation as well as development of cracks on the surface or coloring. In order to overcome such a defect, the use of rubber having almost no double bond, such as acrylic rubber, ethylene-vinyl acetate copolymer rubber, chlorinated polyethylene or ethylene-propylene rubber, instead of diene rubber has been proposed, and to this end, graft copolymer resins made of acrylonitrile, styrene and rubber copolymers have been produced by various polymerization methods such as emulsion polymerization, bulk-suspension polymerization and solution polymerization.
An ethylene-propylene type rubber is insoluble in a polar vinyl monomer such as acrylonitrile, and hence such a rubber will not be dissolved in a mixture of aromatic vinyl and polar vinyl compounds, for example, a mixture of styrene and acrylonitrile, and is only partly swollen even under a high temperature condition. Therefore, the above-said type of graft polymerization has been considered hardly attainable without using an organic solvent for dissolving ethylene-propylene rubber.
There are known methods for producing graft copolymers by using ethylene-propylene rubber as rubber component, as for instance proposed in U.S. Pat. Nos. 3,538,191 and 3,538,190 and Japanese Patent Application Kokoku (Post-Exam Publn) Nos. 32585/1973 and 14874/1974. However, any of these methods is a one-stage process and uses an organic solvent for dissolving ethylenepropylene rubber, so that these methods, although capable of producing graft copolymers with high impact strength, involve troublesome steps for removal and refining of the solvent from the produced resin, and also such steps have possibility of causing coloring of the produced resin.
U.S. Pat. Nos. 3,642,950 and 3,819,765 disclose a graft copolymer producing method according to a two-stage process in which solution polymerization is performed at the first stage, but this method also has the similar problem because of the use of a solvent in the first stage, and the obtained graft copolymers are merely of the same degree of impact strength as the products obtained from the one-stage solution polymerization.
As a method for producing aromatic vinyl compound-ethylene-propylene rubber-polar vinyl compound graft copolymers without using any organic solvent, there has been proposed a technique comprising the steps of dispersing ethylene-propylene rubber in an aqueous medium by using an emulsifier to form a rubber latex and then graft polymerizing the vinyl monomer (Japanese Patent Application Kokoku (Post-Exam Publn) No. 14549/1974). According to such an emulsion polymerization method, however, it is hard to obtain a stable rubber latex and even if a graft polymer could be obtained, such a polymer is low in impact resistance. There is also known a method in which vinyl monomers such as styrene and acrylonitrile are grafted onto ethylene-propylene rubber by way of bulk-suspension polymerization by using a styrene-acrylonitrile copolymer as dispersant (U.S. Pat. No. 3,538,192), but the products obtained from this method are also low in impact strength. It was thus impossible with any of the known methods using no solvent to obtain the graft copolymers having satisfactory impact strength.
There is further known a method in which a styrene-acrylonitrile copolymer is blended with a graft copolymer obtained by grafting styrene, acrylonitrile or the like onto a rubber-like polymer (U.S. Pat. Nos. 3,489,821 and 3,489,822), but the products from this method are also poor in impact resistance.